Discussion

Comment (1)

"Ichnogenous Palaeophycus Hall from the Bagalkot Group, Karnataka State" by K.O. Kulkarni and V.D. Borkar. Jour. Geol. Soc. India, Feb. 1997, Vo1.49, No.2, pp.215-220.

A.V. Jayaprakash writes:

r

would like to differ with the authors' view on their observation regarding the stratigraphic position of the lithounit in which the burrows of Paleophycus Hall have been reported.

The fossil locality given by the authors is almost at the contact of Badami sandstone grading into shale through siltstone and not from the Yadhalli Argillite, a unit which is not seen in the vicinity.

Extensive mapping carried out in the adjacent segments brought to light that the quartzites of Mallikarjuna Gudda is a part of Saundatti Quartzite that forms the basal unit of Bagalkot Group (Jayaprakash et al. 1985). This unit extends further beyond Betgeri upto Gospel and again resurfaces at Mannikeri (Toposheet No. 48-P/4) amidst the vast tracts of Deccan basalts; the nearest outcrop of quartzite in the southwest is around Malgatti (Toposheet No. 47-1/13), probably defining the boundary of the Bagalkot basin, in this sector. Though the quartzites of Mallikarjuna Gudda is in direct contact with the arenites of Badami Group, their boundary can be easily demarcated based on their distinct geomorphic expression, field characters and fabrics under the microscope. The arenites of Badami interbedded with interformational conglomerate show southerly dip due to a local fault. The silty shale in which the ichnofossils are reported, constitute an informal horizon within the Cave Temple Arenite member of the Badami Group.

Based on the assumption that Palaeophycus is associated with the Yadhalli ArgiI1ite, the authors have assigned the lower limit of these burrows to mid-Riphean, contrary to the earlier reports fixingtheir lower limits of age ranging to Vendian.

1 have mapped the lithounit from which the burrows are reported as part of Badami sequence of Vendian age; this age limit corroborates wen with the similar observations by Narbonne et al. (1987) from the Chapel Island Formation of Vendian age in Burnin Peninsula, Canada and else-where in other continents.

Therefore, the lower limit of Palaeophycus cannot be brought down in chronostratigraphic column below 1000 m.y. (i.e mid-Riphean) from available evidences in this sector.

Kantimati Kulkarni and V.D. Borkar reply:

We thank Jayaprakash for his comments. The review article by him (Jayaprakash et al. 1987) was referred by us as the most authentic compilation on the stratigraphy of the Kaladgi Basin which, however, is not free from certain shortcomings. It has to be clarified that for most of the members in the classification proposed by them, the type section has not been specified.

In the area around Mardi Shivapur, the field relationships of various members are obscured by structural disturbances, extremely uneven palaeoslopes of the basement floor and overlying lava flows of the Deccan Trap. However, we have no doubt that Palaeophycus-bearing rock belongs to the Bagalkot Group.

We would like to clarify that the sandstone west of Mardi Shivapur with intraformational conglomerates in not mistaken by us as the Muchkundi Quartzite or the Bevinmatti Conglomerate. We have identified these strata with gentle southerly dips as the Cave Temple Arenite. An arenite unit exposed in the southern parts of the Kaladgi Basin, obviously belonging to the Bagalkot Group (Kale et al. 1996) which succeeds the basal sand-stone-shale-dolomite sequence remains unrecognised in the existing classification of the Kaladgi Supergroup (Jayaprakash et al. 1987). This unit was noted earlier by Foote (1876) and constituted the Ma11ikarjuna Gudda Range. Given the northerly dips of the Palaeophycus-bearing shales, this quartzite must be stratigraphically above them, but certainly older than the capping Badami sandstones. The arenite which we have called Muchkundi Quartzite is the one comprising Mallikarjuna Gudda Range. A distinct band of conglomerate occurs at its base and is exposed immediately to the north of Mardi Shivapur, which we have called as the Bevinmatti Conglomerate. The argillite member yielding Palaeophycus is distinctly older than this arenite. After receiving comments from Jayaprakash, we checked the positions of all the lithounits with the help of our field notes and aerial photographs. The arenite comprising Ma1likarjuna Gudda is not the oldest horizon in the Bagalkot Group and therefore cannot be classified as Saundatti Quartzite. Also. Palaeophycus bearing argillite cannot be mistaken for shale intercalations within, or lateral extension of the Cave Temple Arenite Member.

There are many dug wells present in an extensive area east of Mardi Shivapur, sunk in the argillite member. In none of these wells, including the one from which specimens of Palaeophycus were collected, arenite was encountered below the argillite. The well from which these specimens were obtained, is fairly deep and just a few metres away from the contact of argillite and arenite. Had this arenite been the Saundatti Quartzite, then it should have been definitely seen below the argillite in that well. But it is not so. This confirms that the arenite is not the oldest litho unit in the area around Mardi Shivapur, and therefore, not Saundatti Quartzite.

The arenite constituting the Mallikarjuna Gudda Range together with underlying Palaeophycus bearing argillite represents the Bagalkot Group. They have steep northerly dips and are unconformably overlain by the Cave Temple Arenite Member which shows low southerly dips. It is a representative of the Badami Group.

Thus, the arenite member belonging to the Bagalkot Group occurs just below the unconformity which separates the Bagalkot Group from the Badami Group. Therefore it has to be identified as the Muchkundi Quartzite according to the classification proposed by Jayaprakash et a1. (op. cit) themselves. As regards the argillite member just below it, there is no other option but to regard it as the Yadhalli Argillite. For these reasons we feel justified in modifying the lower.limit of range of Palaeophycus to mid-Riphean.

References

JAYAPRAKASH, A.V., SUNDARAM, V., HANS, S.K. and MISHRA, R.N. (1987). Geology of the Kaladgi-Badami Basin, Kamataka. In: Purana Basins of Peninsular India. Mem. Geot. Soc. India, 6. pp.201-225.

NaRBONNE. G.M .. MYRON. P., LANDING, E., ANDERSON, M.M. (1987). A candidate stratotype for the Precambrian-Cambrian Boundary. Fortune Head, Bumin Peninsula Southeastern Newfoundland. Canadian Jour. Earth Sc. v.24, pp.1277·1293.

Comment (2)

"Review of Precambrian Porphyry Cu±Mo±Au Deposits with special reference to Malanjkhand Porphyry Copper Deposit, Madhya Pradesh, India" by D.B. Sikka and C.E. Nehru, Jour. Geol. Soc. India, March 1997, vo1.49, No.3, pp.239-288.

K.R. Raghu Nandan, 'Venkatadri', 787, 7th Cross, M.C. Layout, Vijayanagar, Bangalore 560040 writes:

Porphyry ore systems and the calc-alkaline rocks that host them are regarded essentially as products of Mesozoic-Cenozoic orogeny and more than half the world's production of copper metal is met by p()rphyry copper deposits. The porphyry copper deposits in older rocks -Archaean and Precambrian, were generally considered to be ill-defined occurrences and sub-economic. The review article by Sikka and Nehru (Jour. Oeol. Soc. India, v.49, pp. 239-288) is a noteworthy contribution as it focuses attention on this not very well known type of deposits by describing their salient features which are of diagnostic value.

I have a few observations to make after reading this interesting paper. The younger porphyry copper deposits (of Phanerozoic age) are no doubt the major source of copper in the world. It may be that metal enrichment had already taken place in certain sectors of the earth's crust before most of the younger deposits originated at the sites of active or once active subduction zones with an intimate genetic relationship between them and calc-alkaline intrusive-extrusive activity. This emphasis on coupled magmatic-tectonic activity (Bowen and Gunatilaka 1997), probably resulted in larger deposits of a better grade and metal concentration than the "older" porphyry copper deposits (of Archaean - Precambrian age) which might not have had the same advantage. Because, in the former type, the metal is considered to have been derived from older are bodies at depth present in the crust beneath the deposit, from the wedge of upper mantle above the Benioff zone, and from subducted rocks (Mitchell and Garson, 1981).

The authors have dealt in detail on the geological environment of Precambrian porphyry copper deposits. The stable isotope studies yield important data regarding their geochemical environment and genesis. It appears that the authors have depended exclusively on strontium isotope studies - initial 87Sr/R6Sr and Rb/Sr ratios. I would like to know whether

6

34S/032

S

ratios of Malanjkhand porphyry copper deposit are available and whether they support derivation from the mantle. Such data is likely to throw light in understanding the physicochemical conditions and environment for the precipitation of sulphides.

In discussing exploration potential for porphyry mineralisation in India, the authors have mentioned certain areas in northern Karnataka (Sugriva Craton of Sikka) as possible targets for exploration for porphyry eu ± Mo ± Au deposits. In my special report (Raghu Nandan 1991, unpublished) on "Copper Resources in Karnataka" (prepared while working in OSI), I wrote

"The area (of about 20,000 sq. km. in north Karnataka) predominantly consisting of younger granites/older gneisses with enclaves of greenstone belts, contains few small well-known copper de-posits (Kallur. Machanur and Thintini) and gold dede-posits (of Hutti, Gadag and Mang]ur). Besides copper and gold, incidences of molybdenum. tungsten and lithium are reported. The geological setting in parts of this region appear to be rather similar to the largest copper deposit known in the Indian Shield. viz., Malanjkhand porphyry type, Madhya Pradesh. This block assumes priority for coverage by mu[tisensor airborne surveys".

Raichur granite (in Dharwar Craton) covering an area of about 8,500 sq. km. is considered to be the equivalent of the Closepet granite (2000 - 2400 Ma). The pluton is largely composed of granite-granodiorite-adamellite. Rare metal assemblages are also reported - Manglnr (Sn-Nb-Ta), Amareshwar (Li), Raichur (U and Y). Recently, occurrence of molybdenite was reported at Ashapura, 6 km SSW of Raichur town (Prakash 1996), in addition to earlier known occurrences at Malatgud and Raichur. Detailed petrological, petrochemical and mineralogical data on Raichur granitoids are lacking. Based on limited information, it is mentioned that Raichur granitoids show polyphase characteristics of mantle/crust origin i.e. within this region both S and I-type granites occur, besides certain transitional types. It may be pertinent to quote the observations from Friend (1994) - "The many styles of emplacement from the large stocks and bosses of the Closepet granite to the myriad granite sheets and veins which intrude some of the greenstone belts, such as that in the Hutti area, provide such a contrast in the ductility state of the crust that several ages and levels of emplacement of granite are represented."

Amongst the known copper occurrences in Raichur granite terrain - Thintini, Kallur, Machanur, Gogalgatti and Antharagange - all explored by the Department of Mines and Geology, Karnataka, I rate Kallur as the most significant/promising prospect. The copper mineralisation is along a shear/breccia zone with low grade disseminated ore enveloping higher grade ore. There are evi-dences to suggest occurrence of a zone of secondary enrichment. There are prominent hematite stains in the area. Native copper is also reported. Alteration observed includes saussuritization, chloritisation and kaolinisation. The continuity of are zone, both laterally and at depth deserves to be explored (Radhakrishna 1996).

Table I of the review paper furnishes salient features of Precambrian porphyry eu ± Mo ± Au deposits. The copper grade in many deposits is between 0.13 and 0.67% (0.85% in Malanjkhand) and in the second largest depQsit at Haib River, Namibia, the grade

is

0.3%. With the develop-ment of solvent extraction - electrowinning (SX-EW) technology, economic recovery of high-purity copper from very low grade deposits is now possible. SX-EW enables leaching of very low grade copper ores - as low as 0.1 % (McLemore 1996). Although high purity copper could be produced by this technology, no gold, silver or molybdenum recovery is possible.

The lowest economically mineable grade of porphyry copper ores has gradually been dropping and large bodies of mineralised porphyry with copper concentrations somewhat below the cutoff grade will almost certainly be'mined during the next century (Holland and Peterson 1995). In this context, it may be expected that copper exploration scenario will have a major shift towards granitoids of late Archaean - Precambrian age and the "older" porphyry copper deposits could prove to be an important source in the world.

References

BOWEN, R. and GUNATILAKA, A., (1977). Copper: Its Geology and Economics. Applied Sc. Pubis. Ltd., London. FRIEND, C.R.L, (1994). Gneiss - granite - chamockite relationships in Kamataka and the Dharwar Craton: A review.

GEOKARNATAKA-MGD Centenary volume., pp.6S~80.

HOLLAND, H.D. and PETERSEN, U., (1995). Living Dangerously, the Earth, its Resources and the Environment. Princeton University Press, Princeton, New Jersey.

MCLEMORE, V,T .• (1996). Copper in New Mexico. New Mexico Geology. Science and Service v.18, Nos.2, pp.25-36. MITCHELL, A.H.G. and GARSON, M.S., (1981) Mineral Deposits and Global Tectonic Settings. Academic Press. PRAKASH, H.S.M., (1996). A note on the occurrence of Molybdenite at Ashapura, Raichur District, Karnataka.

Jour. Geoi. Soc. India, v.48, pp.15-25.

D.B. Sikka and C.E. Nehru reply:

We welcome the comments of Raghu Nandan on the subject of Precambrian porphyry copper deposits in general and especially those relating to the possible occurrence of similar deposits in Karnataka. The information he refers to, unfortunately, is available only to the GSI personnel and not accessible to the general geologic community.

Raghu Nandan's point about 'coupled magmatic-tectonic activity' as applied to Precambrian deposits is not clear, unless he meant that the Precambrian deposits are reworked and enhanced in grade through subsequent, younger tcctonic or magmatic episodes. In our considered opinion, such speculations are premature without specific knowledge of methodology and data. What we need are methods of detailing different episodes of magmatic/tectonic activities and their chrono-logical sequences and the changing ore concentration behaviour. This is a tall order, and only painstaking, detailed structural mapping coupled with geochemical (including isotopic) work can lead to answers. There is a complete dearth of detailed geological maps of mineral deposits under exploitation and exploration 'of various mineralized belts.

The copper occurrence quoted by Raghu Nandan as potential mineral occurrences of eco-nomic significance, an interestingly enough, fall along a zone generally trending north-south and are within the granite plutons of about 2000 million years of age. This 'younger granite' zone, perhaps, defines a tectonic zone of emplacement of granite plutons during the Proterozoic times. This entire zone deserves close attention for potential economic deposits.

We note Raghu Nandan's comments about stable isotopes of sulfur. We have not collected these data. Such information could give us clues to the nature of source materials of the plutonic bodies and the ore deposits. But, this would be the second step. As a first step, we have restricted our investigation to the use of isotope ratio ofR7_Sr/86_{Sr. This ratio is useful in determining areas of}

I-type granitoids which normally host porphyry type Cu±Mo±Au mineralization. It is worth noting that Faure (1986) 'Principles of Isotope Geology', John Wiley, pp.523-549, concludes that attempts to use sulfur isotopes to determine the source of sulfur in sulfide ore deposits is disappointing.

:BOOK REV1LIEW

GEOTHERMAL ENERGY IN INDIA, (1996); Eds. U.L. Pitale and R.N. Padhi, Geological

Survey of India, Special Publication No. 45, Geological Survey of India, 27 Jawaharlal Nehru Road, Calcutta - 700 016, 391pages, Price: Rs.260, Foreign: US $94.

Geothermal energy is a non-conventional, non-renewable but environmentally benign, resource. Early work on the Indian geothermal resources started with a compilation of 99 springs in India and adjacent countries published in the Journal of the Asiatic Society by R. Schlagintweit in 1864. Then followed the monumental work of R.D. Oldham covering 300 springs CGSI Memoir 1882), the review of 43 springs mainly from Bengal, Bihar and Bombay by P.K. Ghosh, (Indian Science Congress 1948), and the grouping of23 springs in the northwest Himalayas on a geotectonic basis by V.S. Krishnaswamy, (Indian Geohydrology, 1965), all from the Geological Survey of India (GS1). The first systematic study of the genesis of hot-springs of the